It is well known to circulate a fluid from a pressurized fluid source, such as hot water for example, through a conduit arranged on or under a surface in order to heat the surface. Building heating systems are known where the conduit is arranged in loops such that the conduit passes back and forth at a spacing of a few inches, and hot water is circulated through the conduit. In a typical application the conduit can be embedded in a concrete floor, or arranged inside a radiant heating panel. Several radiant heating panels are sometimes connected in series such that the fluid circulates a considerable distance before returning to the boiler.
Such systems in a portable configuration are also used in construction projects, for example when thawing frozen ground and curing concrete. Where winter temperatures fall below freezing, ground must often be thawed prior to construction to facilitate excavation. Concrete must also be kept at temperatures above freezing in order to cure properly.
For portable applications such as ground thawing and curing concrete, flexible hoses are typically laid out in a back and forth pattern on the surface, with a spacing of 12-24″. When curing concrete it is also known to embed the hoses in the concrete to increase efficiency by better retaining and distributing the heat in the concrete. These hoses then remain in the finished concrete and are sacrificed, or in some cases are used to heat the finished building by circulating hot water through them. Such a system is described for example in U.S. Pat. No. 5,567,085 to Bruckelmyer.
In typical use, the hose will be from 300 to 1500 feet in length, depending on the ambient temperature, the size of the area to be thawed, the capacity of the boiler, and like considerations. Typically the hoses and the surface being heated will be covered with insulated membranes to retain the heat on the surface. The rate of heating will vary but as an example, ground may typically be thawed at a rate of about one foot of depth per day.
In a typical ground thawing application, fluid at a temperature of 170°-190° F. is pumped from a boiler into the inlet end of the hose, through the looped hose and from the outlet end of the hose back to the boiler. Radiant heat from the fluid passing through the hose is transferred to the surrounding ground or concrete surface. As the fluid flows through the hose, the transfer of heat to the surrounding grounds results in a progressive reduction in the temperature of the fluid at any particular point along the path of flow, such that the fluid exiting the outlet end of the hose will be at a much reduced temperature as low as 80° F.
Since heat transfer is dictated by the difference in temperature between the fluid in the hose and the surrounding ground, the area near where the hot fluid enters the inlet end of the hose at about 180° F. receives more heat than the area near where the cooled fluid exits the outlet end of the hose at 80° F. and returns to the boiler. The end result is that a surface near the inlet end of the hose receives more heat than a surface near the supply end of the hose, and a temperature gradient is induced across the area covered by the hose.
Maintaining the temperature of concrete at a satisfactory level during curing presents increased challenges compared to thawing ground. The American Society for Concrete Contractors recommends that the temperature of the concrete be maintained between 50 and 70° F. As concrete initially contains a significant amount of moisture, it is subject to freezing, which inhibits the initial setting process. In addition, even once the initial setting process has occurred, concrete must be further cured in order that the concrete will achieve its intended strength. Ambient temperature need not even be below freezing in order to comprise the curing process
In areas that experience high ambient temperatures, the concrete may dry too quickly. As happens with concrete that freezes before curing, concrete that is too warm dries too quickly and so suffers from reduced strength and is subject to cracking. In hot climates, ice is sometimes mixed with the concrete to reduce the temperature. Also it is known to circulate carbon dioxide gas through conduits similar to the fluid loops described above in order to cool the concrete.
Proper curing of concrete can affect the final strength by several-fold, and so significant attention is paid to maintaining a desirable temperature and level of hydration of the freshly poured concrete in order that the curing process will be the most effective, and the finished concrete product will display the highest degree of strength. It is thus recommended that fluid line temperatures in a fluid loop system be kept at between 70 and 80° F. while curing concrete.
Since the optimum temperature range for curing concrete is quite narrow compared to a ground thawing application, the difference in the inlet and outlet temperatures of fluid in hoses for curing concrete should be kept to a minimum. Temperature gradients within a slab of concrete result in different curing rates that lead to the creation of physical stress points within the concrete which can manifest as cracks and reduce the overall strength and quality of the concrete
Decreasing the time the fluid is in the hoses or conduits can result in a reduced temperature gradient. To reduce this time the pressurized fluid source is typically connected to supply and return manifolds, and then a plurality of shorter hoses are connected to the manifolds in order to reduce the length of the hoses and thus reduce the temperature drop in the hoses. Also the inlet end of one hose, carrying warmer fluid, can be arranged beside the outlet end of another hose in an attempt to even out the heat transfer. The hoses however must be long enough to reach the farthest end the surface being heated in order to avoid the need for multiple boilers arranged around the surface. Thus instead of a single temperature gradient across the surface, a number of the temperature gradients are created across the surface, and the temperature gradient typically remains significant.
Such manifolds are used as well in permanent applications where a number of radiant heating panels or floor heating sections are each connected to the manifolds such that the length of the circulation path and the resulting temperature drop in the circulating fluid is reduced.
In a portable application, the hoses may also be re-arranged during the process in order to place the hottest portion of the hoses near material that to that point had been near the cooler portion of the hoses and was heating more slowly. This solution requires considerable effort and expense in placing and re-placing the hoses in various patterns required as the operation proceeds, and becomes more problematic when thousands of feet of tubing have to be arranged, a situation common in larger construction projects.
Thus in typical ground thawing applications, where the aim is simply to thaw the ground to the required depth, the apparatus is often simply operated until the entire area of interest is thawed to the desired extent. The result is that by the time the area near the outlet is thawed to the required depth, the area near the inlet is typically thawed to depth much greater than is required. Considerable energy and operational time is therefore wasted.
The longer any particular pocket of fluid is exposed to the surface being heated, the more the temperature of that pocket of fluid will drop. Moving the fluid through the hoses faster means that any particular pocket is exposed for a reduced time, resulting is less temperature drop. The fluid pressure can be increased in order to decrease the time it takes to flow through the hose, however higher pressures require more costly pumps and hoses that are adapted to handle the increased pressure. Such hoses are also not as flexible as lower pressure hoses, and are more difficult to handle and arrange in portable applications. Leaks in a high pressure system could also pose a safety risk.
Similarly increasing the diameter of the hoses means more fluid is exposed to the surface, with the result that less heat is taken out of any individual pocket of fluid, and a reduced temperature gradient can be achieved. Large hoses also allow the fluid to flow faster as with increased pressure. Again such larger hose is more costly than a similar length of smaller diameter hose, as well as being more difficult to transport and handle.